A problem with known crystal controllable oscillator circuits is that of oscillating at overtone frequencies of the controlling crystal.
It is known that oscillation at overtone frequencies may be inhibited by the use of circuits having sensibly lower gain and/or sensibly less favourable phase shifts at the overtone frequency than at the fundamental.
In integrated circuit technology such circuits have been achieved by the use of resistor-capacitor time constants to provide the desired response.
Variations in these time constants due to temperature and manufacturing tolerance effects may cause variations in loop phase shift of the oscillator and consequently variations in frequency.